1. Field of the Invention
The present invention relates to a sheet conveyance device for conveying a sheet and an image forming apparatus or an image reading apparatus provided with the sheet conveyance device.
2. Description of the Related Art
Recently, in the image forming apparatus such as a printer and a copying machine, a sheet detection sensor is arranged to detect a sheet in a sheet conveyance path. The sheet detection sensor detects a position of the sheet conveyed along a guide member constituting the sheet conveyance path. In the image forming apparatus, an image is formed in synchronization with sheet conveyance timing based on the detection result of the sheet detection sensor, which prevents image defects such as image position shift on the sheet. In the case where the sheet cannot be detected at predetermined conveyance timing due to sheet jam in the conveyance path, a sheet conveyance operation is stopped at an early stage to reduce a quantity of user's work for removing the hammed sheet. Additionally, the jammed sheet is prevented from having damage to the apparatus.
There is known a configuration in which, in addition to the sensor, a flexible guide assist member is provided in a guide member constituting the conveyance path in order to restrain the generation of the jam. The guide assist member is provided in the conveyance path while inclined toward a sheet conveyance direction, and the guide assist member prevents the sheet jam by restraining a behavior in a surface direction of the curled sheet having insufficient flatness.
Sometimes the sheet conveyance path in the image forming apparatus is formed by plural conveyance units which are detachably attached to the image forming apparatus. In such cases, sometimes the jam is generated because the conveyance path position of the conveyance unit located on an upstream side in the conveyance direction is shifted from the conveyance path position of the conveyance unit located on a downstream side by a fluctuation of position accuracy. Even in such a configuration, the guide assist member is provided so as to be extended from one conveyance unit to the other conveyance unit, which strains the jam due to the position shift of the conveyance path to perform the good conveyance.
In the case where the apparatus having the plural conveyance units is stopped by detecting the sheet jam, sometimes the apparatus is stopped while the jammed sheet straddling the plural conveyance units. At this point, in the configuration in which each conveyance unit may be drawn in a direction orthogonal to the sheet conveyance direction, when the conveyance unit is drawn in removing the sheet, the sheet is possibly broken between the conveyance units and jammed in the conveyance path, or the guide member constituting the conveyance path is possibly deformed.
Therefore, in order to prevent the above situations, there is disclosed a technique in which a sensor is provided to detect the sheet which is stopped while straddling the conveyance units in addition to the sensor provided in each conveyance unit (for example, see Japanese Patent Laid-Open No. 7-53087). According to the technique disclosed in Japanese Patent Laid-Open No. 7-53087, when the sensor determined that the sheet is stopped while straddling the conveyance units, the conveyance units which the sheet straddles are simultaneously drawn while coupled by a solenoid, and a user performs a jam clearance operation.
A type in which a translucent sensor detects fall of a retractable lever when the sheet abuts on the retractable lever projected in the conveyance path during the sheet conveyance is known as the conventional sheet detection sensor used in the apparatus (hereinafter referred to as lever sensor type). A type in which the presence or absence of the sheet is directly read in a noncontact and optical manner by providing an optical sensor including a light emitting portion and a light acceptance portion in the conveyance path is also known as the conventional sheet detection sensor (hereinafter referred to as optical sensor type).
In the conventional sheet detection sensor, the lever sensor type needs a lever, a translucent type sensor, and a configuration in which the lever and the translucent type sensor are attached into the conveyance path. The optical sensor type needs a sensor for the light emitting portion, a sensor for the light acceptance portion, and a configuration in which the sensors are attached into the conveyance path. Therefore, an installation space is required to attaché the sensors and the configuration in which the sensors are attached into the conveyance path, which blocks downsizing of the apparatus.
In the conventional technique, the sheet detection sensor and the guide assist member are separately disposed in the conveyance path. In order to enhance the accuracy of sheet detection, preferably the sensor is disposed near the guide assist member which restrains flutter of the sheet during the conveyance. However, because the sensor needs the installation space, it is necessary that the sensor be separated away from the guide assist member. Therefore, the sheet is detected in the fluttering state, which worsens the accuracy of detection in the sheet particularly having the bad flatness